The theory behind spent catalyst distributors has been known for many years. Generally, once catalyst has been used in a cracking process, it is referred to as being “spent” catalyst. The spent catalyst is covered with coke, which covers the active sites on the catalyst thereby substantially reducing the activity of the catalyst. It is possible, however, to remove the coke from the catalyst through the use of a regenerator. The spent catalyst is fed into the regenerator and the deposited coke is burned away, leaving only the catalyst. The hot re-activated catalyst is then removed from the regenerator to be reused in the cracking process. It is a known problem in the art that if the spent catalyst is not distributed into the regenerator in a uniform manner, the catalyst is not regenerated as cleanly, after-burning and the associated temperature increase in the dilute phase of the regenerator is present, and NOx is generated in undesirable amounts. Several past patents have attempted to resolve the deficiencies of the prior art distribution problems with varying degrees of success.
For example, U.S. Pat. No. 4,150,090 to Murphy et al. teaches an apparatus that is used in the regeneration of cracking catalyst. Arms which carry spent catalyst are connected to a center shaft and extend outward from the shaft but do not touch the outer walls of the regenerator. This patent thus discloses a shape that is very similar to the spokes on a wheel, when viewed from above. However, this design causes the spent catalyst to be distributed in the regenerator in a very uneven manner.
U.S. Pat. No. 5,635,140 to Miller et al. discloses a design for a spent catalyst distributor for a fluid catalytic cracking unit regenerator. In this design, the spent catalyst is aerated in the spent catalyst distributor by air and combustion or exhaust gases. The aeration facilitates the flow of the spent catalyst through the trough arms of the spent catalyst distributor. As in the Murphy patent, arms that carry spent catalyst are connected to a center shaft and extend outward from the shaft. This patent thus discloses a shape that is very similar to the spokes on a wheel, when viewed from above. However, this design also causes the spent catalyst to be distributed in the regenerator in a very uneven manner.
U.S. Pat. No. 6,809,054 to Myers et al. is directed towards a distributor arrangement for introducing spent fluid catalytic cracking catalyst more uniformly across the dense bed of a regenerator to provided more even contact with regeneration gas in order to avoid hot spots and zones of incomplete combustion. This invention also contemplates the possible use of aeration to fluidize the header to further assist catalyst flow. A spent catalyst-carrying arm extends out from the wall of the regenerator and extends towards, but doesn't touch, the opposite wall. This patent also teaches that small arms can extend from the main arm at angles of 90° or 60°. However, this design, while not in the shape of a spoked wheel, also causes the catalyst to be distributed in the regenerator in a very uneven manner.
Accordingly, it would represent an advancement in the art of regenerating spent catalyst in cracking units if there were a design that would allow the distribution of spent catalyst onto the catalyst bed in a very even manner that would improve the efficiency of the regeneration of catalyst.